The invention relates to a low-pressure mercury discharge lamp which comprises an elongate tubular lamp vessel which is sealed in a vacuumtight manner, which extends in a meander shape parallel to a flat plane, which has an inner surface coated with a fluorescent powder, and which is provided with a lamp filling comprising a rare gas and mercury and with cold electrodes between which a discharge path extends. The invention further relates to an illumination panel, in particular an illumination panel destined for a liquid crystal display, provided with such a low-pressure mercury discharge lamp.
Such a lamp is known from a brochure of the Harison Electric Co, Ltd. The lamp described therein is recommended for use in an illumination panel for liquid crystal displays. The lamp has an external diameter of 6.2 mm (internal diameter approximately 4.3 mm) and the length of the discharge path is approximately 500 mm. This lamp is bent into a meander shape, i.e. the lamp vessel comprises a number of substantially parallel legs which are connected in series with one another and whose centrelines lie in a flat plane. The known lamp is suitable for being incorporated in an illumination panel for a 6" display screen and results in a smaller constructional height of this panel than is possible with similar bent fluorescent lamps of greater lamp diameter. The operating voltage specified for the known lamp is 530 V and the lamp current is no more than 6.0 mA, which is interesting for the use in portable equipment powered by batteries.
The known lamp is not suitable for larger displays of a similar comparatively small constructional height of the panel. To obtain an even illumination of a greater surface area, one would have to have recourse to a number of lamps of the kind described, so that a number of independent discharge paths would be created. An increase in the number of lamps leads to a proportional increase in the electrode losses, which is a disadvantage in a liquid crystal display unit which is often a portable one provided with a battery system. Moreover, an even illumination of the panel cannot be guaranteed in the case of independently arranged lamps because small differences in brightness between the lamps cannot be avoided in practice.